To examine the direct effect of E2 on HIV-infection in an in vitro infection assay, purified CD4+ T-cells (>98% purity) were activated in vitro in the presence or absence of E2 for 3 days and infected with R5 (HIV-1 BaL) or X4 (IIIB) viral strains in parallel. Here, CD4 is linked to HIV infectious disease.